9x^2-y^2=(3x-y)(3x+y), but x^2-2xy-y^2 doesn't factorise rationally. If it had been +y^2 it would have been (x-y)^2. (9x^2-y^2)/(x^2-2xy-y^2) (to make the assumed interpretation clear) is the same as (3x-y)(3x+y)/(x^2-2xy-y^2). Or, if we have +y^2 then it's (3x-y)(3x+y)/(x-y)^2. Arguably, this is no simpler than the original!